1. Field of the Invention
The present invention relates to an optical frequency converter that can be used as a variable-wavelength light source of optical routing devices and the like used as switches in optical communications, and more particularly to an optical frequency converter that can be used as a variable-wavelength light source in which the optical wavelength rapidly stabilizes even when switched at high speed.
2. Description of the Prior Art
In the field of optical communications, methods of distributing input optical signals among a number of transmission lines include the following: 1) a method in which the optical signals are converted into electrical signals and then re-transmitted as optical signals along the corresponding transmission lines and 2) a method in which the optical signals are distributed as they are, with each signal being sent to the transmission line concerned in accordance with differences in the wavelength of the light carrying the signal. It is also known that more information is transmitted using the latter method.
The latter method is described, for example, in Reference (Aoyama et al., “Photonic Networks: Outlook and Technical Issues,” O plus E, vol. 22, No. 11, November 2000, pp. 1456-1470). Optical signals input from a plurality of waveguides that have the same wavelength can be transmitted by a single waveguide by changing the carrier wavelength for each of the signals input. This is a well-known method that is also described by the above reference.
In the latter method in which the optical signals are distributed as they are, a variable-wavelength light source is used. Thus, it can be readily understood that it is desirable to use a variable-wavelength light source that is capable of high-speed operation and also has stable characteristics.
Light sources that have conventionally been used for this purpose include distributed Bragg reflection (DBR) lasers and distributed feedback (DFB) lasers. However, after a wavelength has been changed, it takes several tens of milliseconds for the output wavelength to stabilize, and in the case of 40-gigabit/second optical communications in which data is transmitted in 4000-bit packets, the time required to stabilize is longer than the time it takes to transmit one packet (100 ns), posing an obstacle to such high-speed communications.
The present invention can be used for the above purpose, and relates to a variable-wavelength light source that uses a high-frequency signal to convert an optical frequency. Prior-art examples of such a light source are described below.
There are a number of known ways converters work to convert the frequency of an optical input. These include (1) the input of two types of light to non-linear optical crystal to mix the two lightwaves. This is already well known, and is also used for doubling laser frequencies. Also included is (2) a method using a mode-locked laser, comprising using an optical modulator, isolator and Fabry-Perot etalon provided in a laser resonator to generate light pulses. This is also known as a method for generating a sideband of frequency fp that is Km times higher than phase modulation frequency fm (fp=Km*fm). There is also (3) a method comprising modulating the light with a high-frequency signal to derive a sideband that is used to convert the optical frequency.
Using these methods, lightwaves are converted to different frequencies as follows. In the case of the above (1), at least one of the lightwaves is changed to light of a different frequency. In the case of (2), a filter is also provided to select a generated sideband in order to change the light to light of a different frequency. In the case of (3), the frequency of the high-frequency signal is changed. Thus, it can perhaps be readily seen that such methods can be used to change optical frequencies.
The present invention is partly similar to (3) in which the lightwave is modulated by a high-frequency signal to obtain a sideband for converting the frequency. This is explained below.
Using a high-frequency signal to modulate a lightwave is usually accomplished by inputting the optical carrier wave and high-frequency signal to an optical modulator and performing intensity modulation or phase modulation or the like. With this method, when a sideband is obtained having a frequency higher than that of the applied high-frequency signal, the high-frequency signal is multiplied, producing an electrical signal of an even higher frequency that is used for the modulation. Even when the high-frequency signal is thus multiplied, the maximum modulation frequency is limited by the upper limit of the electrical signal. Thus, multiplication or amplification of an electrical signal has been subjected to the maximum frequency limitation of the electrical circuit concerned. There has therefore been a need for a means of overcoming this.
There have been reports of using phase modulation with a high modulation index in an attempt to obtain a sideband having a higher frequency than that of the applied high-frequency signal. One such report (Reference 1: “Generation of ultrashort light pulses using a domain-inversion external phase modulator,” by Tetsuro Kobayashi, Applied Physics, vol. 67, No. 9 (1998), pp. 1056-1060) stated that, with a modulation index of 87 radian, applying a 16.26-GHz high-frequency signal to an optical modulator with a strip-line resonator provided on a waveguide formed of electro-optical LiTaO3 crystal resulted in a spectral width of around 2.9 THz.
Reference 2 (U.S. Pat. No. 5,040,865) describes the method of using a high-frequency signal to modulate monochromatic light with a modulator having nonlinear characteristics, producing a high-order sideband that, by using an optical detector to detect the optical signal, is used to produce a high-frequency signal. The disclosure describes using a first modulator to generate a first high-frequency signal by the above method, applying the signal to a second modulator to use the method to perform modulation with a second high-frequency signal. However, since this uses an electrical signal that is multiplied by an applied high-frequency signal, it is subject to the frequency constraints of the circuit.
To perform the above-described phase modulation using a high modulation index, it is necessary to realize a high modulation index. With this being the aim, in order to increase the amplitude of the high-frequency signal, a strip-line resonator is used as a modulator electrode, which makes it difficult to change the modulation frequency. If, to avoid this, the resonator is not used as the electrode, a high-amplitude high-frequency signal becomes a requirement, so the high-frequency signal is amplified. While it might seem that in this case, it is an easy way of changing the optical frequency by changing the modulation frequency, it is well known that the bandwidth of the amplifier determines upper frequency limit of the modulation signal and the obtained light frequency.
An object of the present invention is to provide an optical frequency converter that has a configuration that makes it possible to obtain high-order sidebands with a high-frequency signal having a lower amplitude than that of the above means of phase modulation using a high modulation index, enabling conversion over a wide range of frequencies even with a low-amplitude high-frequency signal.